Piranhas and tetras share a 2,000-species Neotropical order
Characiformes gathers characins and allies across 28 families and more than 2,000 species, from aquarium tetras to infamous piranhas. Diversity peaks in Neotropical fresh waters. Most are small shoaling forage fish; a few grow large enough for food and sport fisheries.
Characins share a Weberian apparatus, a chain of small bones linking the swim bladder to the inner ear, which places them in the Otophysi alongside the Cypriniformes, Siluriformes and Gymnotiformes. They look rather like carps but many have a little fleshy adipose fin set behind the dorsal fin, ahead of the tail. Their bodies are almost always clad in distinct scales, and because many are carnivorous, most have teeth in a mouth that usually cannot be thrust forward.
Size spans a remarkable range. Africa's goliath tigerfish, South America's golden dourado and the wolffish Hoplias aimara all exceed 1 metre, yet many species stay below 3 centimetres, and the Bolivian pygmy blue characin tops out at roughly 1.4 to 1.7 centimetres. At least 209 species live in Africa, among them distichodontids, citharinids, alestids and the African pikes; the rest are American. Small, colourful, hardy tetras suit community aquariums because they tolerate other fish.
Classification keeps shifting. All characins once sat in a single family, Characidae, from which 18 families have since been split; a 2011 study confirmed a narrower Characidae as a natural group. The African suborder Citharinoidei branched off very early, perhaps in the Early Cretaceous, and some researchers would make it an order of its own. Current lists include the Serrasalmidae, home of the piranhas, the toothless Curimatidae and the freshwater hatchetfishes.
The order probably arose in West Gondwana, the joined landmass of Africa and South America, during the Cretaceous; the opening of the Atlantic may explain why diversity is so much higher on the American side. The oldest accepted fossils come from Late Cretaceous freshwater deposits in Uzbekistan and Hungary, while Brazil's Santanichthys, once cited as a marine ancestor, now looks like a more basal fish.
Source: Characiformes
